The present invention relates to a dielectric filter to be suitable used as a band-pass filter or the like.
As is well known, such a dielectric filter operating as a band-pass filter has a pass bandwidth which is defined by its coupling coefficient. The coupling coefficient of the dielectric filter can be modified to some extent by changing the distance separating its resonators and/or arranging a coupling adjusting hole on it. However, either the distance between the resonators needs to be made extremely small or a very large coupling adjusting hole has to be formed in order to achieve a large coupling coefficient. Then, the filter will show a disproportionally small space separating the resonators or the resonators and the coupling adjusting hole. The process of manufacturing such a filter will inevitably become complicated and an extremely enhanced level of precision machining will become necessary if such a filter has to be down-shed. Dielectric filters of various types have been proposed to overcome these problems by arranging a coupling capacitor on the outer surface of the dielectric substrate of the filter in order to modify the coupling capacitance of the filter.
For instance, Japanese Patent Kokai No. 59-114902 discloses a dielectric filter realized by introducing a plurality of conductors operating as so many resonators into respective holes bored through a dielectric block from a side to the other, an end of each of the conductors being connected to a common conductive film arranged on the outer surface of the dielectric block to form a short-circuit terminal, the other end being left free to form an open-circuit terminal, and arranging a capacitor close to said open-circuit terminals on the surface of said dielectric block to realize capacitance-coupling of said open-circuit terminals of the conductors so that any desired coupling capacitance can be produced for the resonators by appropriately selecting the capacitance of the capacitor.
On the other hand, Japanese Patent Publication No. 3-40962 (U.S. Pat. No. 4,673,902) discloses a dielectric filter realized by introducing internal conductors operating as so many resonators into respective holes bored through a cubic dielectric block from a side to the other, short-circuiting an end of each of said internal conductors by forming an external conductor film as a short-circuit terminal plane on the side of the dielectric block exposing said ends of the internal conductors, leaving the opposite side of the dielectric block exposing said other ends of the internal conductors as an open-circuit terminal plane, a capacitor electrode being disposed on said open-circuit terminal plane for capacitance coupling.
It should be noted that the coupling capacitance between the input/output section and the resonators of a conventional dielectric filter of any of the above types is defined by the capacitance of the capacitor arranged on the outer surface of the dielectric block of the filter and comprising a first electrode disposed on or electrically connected to the open-circuit terminal plane and a second electrode disposed adjacent to said first electrode, said electrodes being normally located on a surface where no external conductor of the dielectric block is found, so that the electrodes are subjected to a certain dimensional limit. More specifically, while the electrodes may be made large by using a considerably large dielectric block, such a large dielectric block may not feasibly be used for a down-sized dielectric filter of any of the types under consideration. Thus, the known techniques can only provide a dielectric filter with a small coupling capacitance and an extremely high sensitivity to external electromagnetic fields and precision machining operations will become necessary if a coupling capacitance of a desired level needs to be realized for the electrodes to be formed.
With any of the above described known dielectric filters, also, it is difficult to polarize the attenuation band of the filter. Incidentally, known techniques for polarization of the attenuation band of a dielectric filter include the one where adjacent open-circuit terminals of dielectric resonators are connected with each other by way of respective reactance devices arranged on the outer surface of the filter and a parallel resonance circuit is formed among the reactance devices and held in an anti-resonance state at a specific frequency in order to polarize the attenuation band (e.g., Japanese Patent Kokai No. 63-60603) and the one where an external conductor and internal conductors are arranged respectively on the outer surface of a dielectric block and the inner peripheral surfaces of holes bored through the dielectric block and grooves cut along strips separating adjacent internal conductors are covered by a conductive film electrically connected to the external conductor, said internal conductors being mutually coupled by different polarities in order to polarize the attenuation band (see Japanese Patent kokai No. 63-90202). However, a dielectric filter realized by using the former technique is excessively sensitive to external electromagnetic fields because a reactance device is formed on the free terminal plane of the filter, whereas one realized by means of the latter technique has a very complicated configuration and therefore requires exquisitely fine machining operations, practically prohibiting any attempt to meet the requirement of down-sizing.
All in all, with any of the known techniques, it is difficult, if not impossible, to provide a down-sized dielectric filter having a polarized attenuation band and dielectric filters prepared by known techniques are normally excessively sensitive to external electromagnetic fields in operation. Additionally such devices require a high degree of precision machining to raise the overall manufacturing cost.